Friday, December 11. 2015

Important notice: After I published this text Adam Langley pointed out that a major assumption is wrong: Android 2.2 actually has no problems with SHA256-signed certificates. I checked this myself and in an emulated Android 2.2 instance I was able to connect to a site with a SHA256-signed certificate. I apologize for that error, I trusted the Cloudflare blog post on that. This whole text was written with that assumption in mind, so it's hard to change without rewriting it from scratch. I have marked the parts that are likely to be questioned. Most of it is still true and Android 2 has a problematic TLS stack (no SNI), but the specific claim regarding SHA256-certificates seems wrong.

This week both Cloudflare and Facebook announced that they want to delay the deprecation of certificates signed with the SHA1 algorithm. This spurred some hot debates whether or not this is a good idea – with two seemingly good causes: On the one side people want to improve security, on the other side access to webpages should remain possible for users of old devices, many of them living in poor countries. I want to give some background on the issue and ask why that unfortunate situation happened in the first place, because I think it highlights some of the most important challenges in the TLS space and more generally in IT security.

SHA1 broken since 2005

The SHA1 algorithm is a cryptographic hash algorithm and it has been know for quite some time that its security isn't great. In 2005 the Chinese researcher Wang Xiaoyun published an attack that would allow to create a collision for SHA1. The attack wasn't practically tested, because it is quite expensive to do so, but it was clear that a financially powerful adversary would be able to perform such an attack. A year before the even older hash function MD5 was broken practically, in 2008 this led to a practical attack against the issuance of TLS certificates. In the past years browsers pushed for the deprecation of SHA1 certificates and it was agreed that starting January 2016 no more certificates signed with SHA1 must be issued, instead the stronger algorithm SHA256 should be used. Many felt this was already far too late, given that it's been ten years since we knew that SHA1 is broken.

A few weeks before the SHA1 deadline Cloudflare and Facebook now question this deprecation plan. They have some strong arguments. According to Cloudflare's numbers there is still a significant number of users that use browsers without support for SHA256-certificates. And those users are primarily in relatively poor, repressive or war-ridden countries. The top three on the list are China, Cameroon and Yemen. Their argument, which is hard to argue with, is that cutting of SHA1 support will primarily affect the poorest users.

Cloudflare and Facebook propose a new mechanism to get legacy validated certificates. These certificates should only be issued to site operators that will use a technology to separate users based on their TLS handshake and only show the SHA1 certificate to those that use an older browser. Facebook already published the code to do that, Cloudflare also announced that they will release the code of their implementation. Right now it's still possible to get SHA1 certificates, therefore those companies could just register them now and use them for three years to come. Asking for this legacy validation process indicates that Cloudflare and Facebook don't see this as a short-term workaround, instead they seem to expect that this will be a solution they use for years to come, without any decided end date.

It's a tough question whether or not this is a good idea. But I want to ask a different question: Why do we have this problem in the first place, why is it hard to fix and what can we do to prevent similar things from happening in the future? One thing is remarkable about this problem: It's a software problem. In theory software can be patched and the solution to a software problem is to update the software. So why can't we just provide updates and get rid of these legacy problems?

Windows XP and Android Froyo

According to Cloudflare there are two main reason why so many users can't use sites with SHA256 certificates: Windows XP and old versions of Android (SHA256 support was added in Android 2.3, so this affects mostly Android 2.2 aka Froyo). We all know that Windows XP shouldn't be used any more, that its support has ended in 2014. But that clearly clashes with realities. People continue using old systems and Windows XP is still alive in many countries, especially in China.

But I'm inclined to say that Windows XP is probably the smaller problem here. With Service Pack 3 Windows XP introduced support for SHA256 certificates. By using an alternative browser (Firefox is still supported on Windows XP if you install SP3) it is even possible to have a relatively safe browsing experience. I'm not saying that I recommend it, but given the circumstances advising people how to update their machines and to install an alternative browser can party provide a way to reduce the reliance on broken algorithms.

The Updatability Gap

But the problem with Android is much, much worse, and I think this brings us to probably the biggest problem in IT security we have today. For years one of the most important messages to users in IT security was: Keep your software up to date. But at the same time the industry has created new software ecosystems where very often that just isn't an option.

In the Android case Google says that it's the responsibility of device vendors and carriers to deliver security updates. The dismal reality is that in most cases they just don't do that. But even if device vendors are willing to provide updates it usually only happens for a very short time frame. Google only supports the latest two Android major versions. For them Android 2.2 is ancient history, but for a significant portion of users it is still the operating system they use.

What we have here is a huge gap between the time frame devices get security updates and the time frame users use these devices. And pretty much everything tells us that the vendors in the Internet of Things ignore these problems even more and the updatability gap will become larger. Many became accustomed to the idea that phones get only used for a year, but it's hard to imagine how that's going to work for a fridge. What's worse: Whether you look at phones or other devices, they often actively try to prevent users from replacing the software on their own.

This is a hard problem to tackle, but it's probably the biggest problem IT security is facing in the upcoming years. We need to get a working concept for updates – a concept that matches the real world use of devices.

Substandard TLS implementations

But there's another part of the SHA1 deprecation story. As I wrote above since 2005 it was clear that SHA1 needs to go away. That was three years before Android was even published. But in 2010 Android still wasn't capable of supporting SHA256 certificates. Google has to take a large part of the blame here. While these days they are at the forefront of deploying high quality and up to date TLS stacks, they shipped a substandard and outdated TLS implementation in Android 2. (Another problem is that all Android 2 versions don't support Server Name Indication, a technology that allows to use different certificates for different hosts on one IP address.)

This is not the first such problem we are facing. With the POODLE vulnerability it became clear that the old SSL version 3 is broken beyond repair and it's impossible to use it safely. The only option was to deprecate it. However doing so was painful, because a lot of devices out there didn't support better protocols. The successor protocol TLS 1.0 (SSL/TLS versions are confusing, I know) was released in 1999. But the problem wasn't that people were using devices older than 1999. The problem was that many vendors shipped devices and software that only supported SSLv3 in recent years.

One example was Windows Phone 7. In 2011 this was the operating system on Microsoft's and Nokia's flagship product, the Lumia 800. Its mail client is unable to connect to servers not supporting SSLv3. It is just inexcusable that in 2011 Microsoft shipped a product which only supported a protocol that was deprecated 12 years earlier. It's even more inexcusable that they refused to fix it later, because it only came to light when Windows Phone 7 was already out of support.

The takeaway from this is that sloppiness from the past can bite you many years later. And this is what we're seeing with Android 2.2 now.

But you might think given these experiences this has stopped today. It hasn't. The largest deployer of substandard TLS implementations these days is Apple. Up until recently (before El Capitan) Safari on OS X didn't support any authenticated encryption cipher suites with AES-GCM and relied purely on the CBC block mode. The CBC cipher suites are a hot candidate for the next deprecation plan, because 2013 the http://www.isg.rhul.ac.uk/tls/Lucky13.html Lucky 13 attack has shown that they are really hard to implement safely. The situation for applications other than the browser (Mail etc.) is even worse on Apple devices. They only support the long deprecated TLS 1.0 protocol – and that's still the case on their latest systems.

There is widespread agreement in the TLS and cryptography community that the only really safe way to use TLS these days is TLS 1.2 with a cipher suite using forward secrecy and authenticated encryption (AES-GCM is the only standardized option for that right now, however ChaCha20/Poly1304 will come soon).

Conclusions

For the specific case of the SHA1 deprecation I would propose the following: Cloudflare and Facebook should go ahead with their handshake workaround for the next years, as long as their current certificates are valid. But this time should be used to find solutions. Reach out to those users visiting your sites and try to understand what could be done to fix the situation. For the Windows XP users this is relatively easy – help them updating their machines and preferably install another browser, most likely that'd be Firefox. For Android there is probably no easy solution, but we have some of the largest Internet companies involved here. Please seriously ask the question: Is it possible to retrofit Android 2.2 with a reasonable TLS stack? What ways are there to get that onto the devices? Is it possible to install a browser app with its own TLS stack on at least some of those devices? This probably doesn't work in most cases, because on many cheap phones there just isn't enough space to install large apps. In the long term I hope that the tech community will start tackling the updatability problem.

In the TLS space I think we need to make sure that no more substandard TLS deployments get shipped today. Point out the vendors that do so and pressure them to stop. It wasn't acceptable in 2010 to ship TLS with long-known problems and it isn't acceptable today.

Wednesday, January 5. 2011

If you've read my last blog entry, you saw that I was struggling a bit with the fact that I was unable to create a PGP key without SHA-1. This is a bit tricky, as there are various places where hash functions are used within a pgp key:

1. The key self-signatures and signatures on other keys. Every key has user IDs that are signed with the master key itself. This is to proofe that the names and mail adresses in the key belong to the keyholder itself and can't be replaced my a malicous attacker.
2. The signatures on messages, for example E-Mails.
3. The preference in side the key - this indicates to other people what sigature algorithms you would prefer if they send messages to you.
4. The fingerprint.

1 is controlled by the setting cert-digest-algo in the file gpg.conf (for both self-signatures and signatures to other keys). 2 is controlled by the setting personal-digest-preferences. So you should add these two lines to your gpg.conf, preferrably before you create your own key (if you intend to create one, don't bother if you want to stick with your current one):

personal-digest-preferences SHA256cert-digest-algo SHA256

3 defaults to SHA256 if you generate your key with a recent GnuPG version. You can check it with gpg --edit-key [your key ID] and then showpref. For 4, I think it can't be changed at all (though I think it doesn't mean a security threat for collission attacks - still it should be changed at some point).

It is also not really trivial to check the used algorithms. For message signatures, if you verify them with gpg -v --verify [filename]. For key signatures, I found no option to do that - but a workaround: Export the key whose signatures you'd like to check gpg --export --armor [key ID] > filename.asc. Then parse the exported file with gpg -vv filename.asc. It'll show you blocks like this:

The digest algo 8 is what you're looking for. 1 means MD5, 2 means SHA1, 8 means SHA256. Other values can be looked up in include/cipher.h in the source code. No, that's not user friendly. But I found no easier way.

The big question remains: Why is this so complicated and why isn't gnupg just defaulting to SHA256? I don't know the answer.

Sunday, December 26. 2010

Having used my PGP key 3DBD3B20 for almost eight years, it's finally time for a new one: 4F9F43A9. The old primary key was a 1024 bit DSA key, which had two drawbacks:
1. 1024 bit keys for DLP or factoring based algorithms are considered insecure.
2. It's impossible to set the used hash algorithm to anything beyond SHA-1.

My new key has 4096 bits key size (2048 bit is the default of GnuPG since 2.0.13 and should be fairly enough, but I wanted some extra security) and the default hash algorithm preference is SHA-256. I had to make a couple of decisions for my name in the key:
1. I'm usually called Hanno, but my real/official name is Johannes.
2. My surname has a special character (ö) in it, which can be represented as oe.

In my previous keys, I've mixed this. I decided against this for the new key, because both my inofficial prename Hanno and my umlaut-converted surname Boeck are part of my mail adress, so people should still be able to find my key if they're searching for that.

Another decision was the time I wanted my key to be valid. I've decided to give it an expiration date, but a fairly long one: 10 years from now.

I've signed my new key with my old key, so if you've signed my old one, you should be able to verify the new one. I leave it up to you if you decide to sign my new key or if you want to re-new the signing procedure. I'll start from scratch and won't sign any keys I've signed with the old key automatically with the new one. If you want to key-sign with me, you may find me on the 27C3 within the next days.

My old key will be valid for a while, at some time in the future I'll probably revoke it.

Update: I just found out that having a key without SHA-1 is trickier than I thought. The self-signatures were still SHA-1. I could re-do the self-signatures and revoke the old ones, but that'd clutter the key with a lot of useless cruft and as the new key wasn't around long and didn't get any signatures I couldn't get easily again, I decided to start over again: The new key is BBB51E42 and the other one will be revoked.
I'll write another blog entry to document how you can create your own SHA-256 only key.

Monday, February 1. 2010

At least since 2005 it's well known that the cryptographic hash function SHA1 is seriously flawed and it's only a matter of time until it will be broken. However, it's still widely used and it can be expected that it'll be used long enough to allow real world attacks (as it happened with MD5 before). The NIST (the US National Institute of Standards and Technology) suggests not to use SHA1 after 2010, the german BSI (Bundesamt für Sicherheit in der Informationstechnik) says they should've been fadet out by the end of 2009.

The probably most widely used encryption protocol is SSL. It is a protocol that can operate on top of many other internet protocols and is for example widely used for banking accounts.

As SSL is a pretty complex protocol, it needs hash functions at various places, here I'm just looking at one of them. The signatures created by the certificate authorities. Every SSL certificate is signed by a CA, even if you generate SSL certificates yourself, they are self-signed, meaning that the certificate itself is it's own CA. From what I know, despite the suggestions mentioned above no big CA will give you certificates signed with anything better than SHA1. You can check this with:openssl x509 -text -in [your ssl certificate]
Look for "Signature Algorithm". It'll most likely say sha1WithRSAEncryption. If your CA is good, it'll show sha256WithRSAEncryption. If your CA is really bad, it may show md5WithRSAEncryption.

When asking for SHA256 support, you often get the answer that the software still has problems, it's not ready yet. When asking for more information I never got answers. So I tried it myself. On an up-to-date apache webserver with mod_ssl, it was no problem to install a SHA256 signed certificate based on a SHA256 signed test CA. All browsers I've tried (Firefox 3.6, Konqueror 4.3.5, Opera 10.10, IE8 and even IE6) had no problem with it. You can check it out at https://sha2.hboeck.de/. You will get a certificate warning (obviously, as it's signed by my own test CA), but you'll be able to view the page. If you want to test it without warnings, you can also import the CA certificate.

I'd be interested if this causes any problems (on server or on client side), so please leave a comment if you are aware of any incompatibilities.

Update 2: StartSSL wrote me that they tried providing SHA256-certificates about a year ago and had too many problems - it wasn't very specific but they mentioned that earlier Windows XP and Windows 2003 Server versions may have problems.

I wrote some articles about hash collisions a while back. Short introduction: A cryptographic hash-function is a function where you can put in any data and you'll get a unique, fixed-size value. »unique« in this case scenario means that it's very hard to calculate two different strings matching to the same hash value. If you can do that, the function should be considered broken.

The MD5 function got broken some years back (2004) and it's more or less a question of time when the same will happen to SHA1. There have been scientific results claiming that an attacker with enough money could easily create a supercomputer able to create collisions on SHA1. The evil thing is: Due to the design of both functions, if you have one collision, you can create many more easily.

Although those facts are well known, SHA1 is still widely used (just have a look at your SSL connections or at the way the PGP web of trust works) and MD5 isn't dead either. The fact that a well-known piece of software got issues depending on hash collisions should raise attention. Pretty much all security considerations on cryptographic protocols rely on the collision resistance of hash functions.

The NIST plans to define new hash functions until 2012, until then it's probably a safe choice to stick with SHA256 or SHA512.